As is well known, various parts of certain vehicles require the use of compressed fluid, such as compressed air, for their operation. For example, the brake systems of trucks and other large vehicles often use compressed air to bias various braking mechanisms, such as spring-based actuators, into braking and non-braking positions.
Accordingly, piston compressors are generally known in the art for generating this compressed fluid. These compressors typically employ a cylinder block with a plurality of piston channels. A plurality of pistons are slidably disposed in the piston channels and are coupled to some mechanism, such as a crankshaft, for causing them to reciprocate back and forth within the piston channels, thereby alternately creating suction and compression strokes. As the pistons reciprocate within the piston channels of the cylinder block, they alternately draw fluid to be compressed into the channels, and subsequently compress and discharge this fluid.
In order to control the introduction and discharge of fluid to and from the piston channels, these compressors also typically include a compressor head having inlet and outlet ports, as well as inflow and outflow channels connecting the ports to the piston channels. Additionally, the compressor heads often include valves, or else separate valve plates are disposed between the compressor head and the cylinder block having such valves, which regulate the inflow and outflow of the fluid. These valves permit the piston channels to communicate with the inlet and outlet channels, and ultimately, the inlet and outlet ports, in the compressor head.
However, one problem with these compressors is that the walls of the piston channels will sometimes become distorted during assembly of the compressor. As previously noted, a cylinder head is typically mounted to the cylinder block by a set of fasteners, usually by inserting a series of bolts through the cylinder head and into threaded openings in the cylinder block. As the head and the block are clamped together by tightening the bolts, the clamping force will often distort the walls of the piston channels adjacent thereto. Uneven application of the tightening forces to the various bolts positioned around the block can further exacerbate this distortion. Additionally, the channel wall with which the piston makes contact as it slides along the channel is often a liner placed within a cylinder bore, which is typically even more prone to distortion.
Distortion of the piston channel walls is a serious problem, as it affects the seal between the piston and the channel wall. For example, often the pistons will include a set of compression rings coupled to the piston head, the annular shape of which engages the cylindrical shape of the piston channel and thereby prevents the passage of oil from the compressor to the fluid compression chambers above the piston head. If the channel wall becomes distorted, and thus, the channel wall is no longer cylindrical, the engagement between the compression rings and the channel wall is less than perfect. This loss of a continuous seal will result in leakage of oil into the compression chamber above the cylinder, which can contaminate the compressed fluid and can affect the components downstream of the compressor. For example, often, an air dryer is used in conjunction with the compressor to remove moisture in the air being supplied by the compressor before it is supplied to the relevant parts of the vehicle. If oil leaks into the compression chamber above the piston head, this oil will contaminate the air dryer system when the compressed air is communicated to it. As another example, if the leaked oil proceeds to the air control valves, it can prevent them from working properly.
Similarly, the piston heads in these types of compressors are also commonly fitted with oil scraper rings that contact the walls of the piston channels. After oil has been deposited on the channel walls due to the exposure of those walls to the oil during the pistons' upward compression strokes, the oil scraper rings serve to scrape the oil off of the walls during the pistons' downward suction strokes, thereby helping to ensure that this oil does not ultimately end up in the compression chambers above the piston heads. However, when the piston channel walls become distorted, the scraper rings, much like the compression rings, are effectively lifted off of the surface of the channel wall, thereby decreasing their ability to scrape the oil therefrom.
A related problem that exists with these types of compressors is that, as the pistons slide within the piston channels, the continuous sliding contact made between part of the piston and the wall of the channel causes the channel wall to heat up. This increase in temperature causes the channel walls to be even more prone to distortion as a result of additional stresses placed on the channel walls by the clamping force of the bolts.
Another problem that results from these types of compressors is that, even if the heat resulting from the friction between the piston and the channel wall does not cause the channel wall to become deformed, it ends up heating the air that is being compressed at the top of the piston channel, which can lead to numerous problems. For example, as previously mentioned, an air dryer is often used in conjunction with the compressor to remove moisture in the air being supplied by the compressor to the relevant parts of the vehicle. Because the air is hotter, it is able to hold more water vapor, and therefore, the air dryer must work harder to remove the moisture. Another problem created by this additional heat is that it causes oil to be more prone to “coking up”—burning and leaving behind carbon deposits. Yet another problem caused by excessive amounts of very hot air is that components of the compressor, and downstream from the compressor, will tend to have a shorter life, in part because of contraction and expansion of those parts from unnecessary levels of heating and cooling.
What is desired, therefore, is a piston compressor where the walls of the piston channels do not become easily deformed. What is further desired is a piston compressor where the walls of the piston channels do not become excessively hot.